1. Field of the Invention
The present invention relates to a signal selector and, more particularly, to a signal selector suitable for a programmed exposure control apparatus (automatic exposure apparatus) of a camera.
2. Description of the Prior Art
Conventionally, an automatic exposure apparatus is proposed to automatically control a shutter speed (a time value by an APEX notation). According to this apparatus, a stop value (an aperture value by the APEX notation) is automatically set in accordance with object brightness (a luminance value by the APEX notation) and a shutter speed preset by object brightness or external manual operation, and the stop value and the object brightness are used to determine whether or not the preset shutter speed is properly determined to obtain a proper exposure. If not, the preset shutter speed is automatically changed by the automatic exposure apparatus so as to obtain a proper shutter speed. More specifically, according to this apparatus, if a proper exposure cannot be obtained even if full-aperture metering is performed or a minimum aperture diameter is obtained corresponding to the preset stop value, the preset shutter speed is automatically corrected to obtain a proper exposure.
The above-mentioned apparatus may be effectively used in either a programmed automatic exposure apparatus or a shutter-priority automatic exposure apparatus. An indicator unit for the automatic exposure apparatus is disclosed in U.S. Pat. No. 4,204,755 wherein a shutter speed is automatically controlled with respect to a minimum stop value and a maximum stop value and an intermediate stop value therebetween.
In the indicator unit described above, however, automatic selector means for selecting a shutter speed comprises a comparator, an analog switch, a logic circuit and the like. Therefore, the number of circuit elements is greatly increased. Since there is a tendency these days to use electronic techniques to achieve a variety of camera functions, it is inevitable that the number of circuit elements of the automatic selector means should increase in the case of arranging an electronic circuit on an IC.
In general, in the process for manufacturing an IC, a simple comparator or a simple analog switch (follower amplifier) requires at least 10 transistors. A logic circuit requires two transistors for each gate. Furthermore, the follower amplifier used as the analog switch requires a capacitor for preventing spurious oscillation. A large area is used for a p-n junction of the diode so as to obtain a sufficient capacitance. For example, the area of the p-n junction corresponds to that of the follower amplifier. For these reasons, a conventional automatic selector means requires about 60 transistors and three capacitors, resulting in a large size IC. This drawback also occurs when an automatic selector means is constituted by discrete electronic elements.